In order to meet increasingly stringent fuel consumption and emission regulations, more attentions are paid to improve engine efficiency. A large amount of energy-saving technologies have been applied in automotive field especially in gasoline engines. It is well known that lean burn and ultra-high compression ratio technologies are two basic and important methods to increase efficiency. In this paper, a rapid compression machine was employed to study combustion process of lean iso-octane mixture at ultra-high compression ratios (16 to 19:1). Regardless of flammability of the mixture, spark was triggered at the timing right after the end of compression, then, the flame propagation and/or auto-ignition can be recorded using high-speed photography simultaneously. The effects of equivalence ratio (φ), compression ratio (ε), dilution ratio, and effective temperature (Teff) on the combustion process was investigated.

The results indicate that the controlled auto-ignition, induced by flame under high pressure and medium temperature in the lean mixture, can reduce combustion duration substantially and meanwhile depress the excessive pressure rise rate. Under φ = 0.4, there is an appropriate temperature range Teff = 840-870 K and pressure range Peff = 25-35 bar to maintain the advantages of flame and auto-ignition interactions which might provide a promising way to achieve higher efficiency using ultra-high compression ratios and lean mixture during engine operation.